The R-T-B based sintered magnet shows excellent magnetic properties and thus is used in the voice coil motor (VCM) in a hard disk drive, various motors such as the motor equipped in a hybrid electric vehicle, household electrical appliances or the like.
Researches and developments have been actively conducted to improve the magnetic properties of the R-T-B based sintered magnet. For example, in Patent Document 1, it has been reported that the magnetic properties can be enhanced and the conditions for thermal treatment also can be improved by adding 0.02 to 0.5 at % of Cu into the R-T-B based rare earth based permanent magnet. However, the method described in Patent Document 1 cannot achieve sufficiently high magnetic properties required in a magnet with good performance such as a high coercivity (HcJ) and a high residual magnetic flux density (Br).
In order to turn the R-T-B based sintered magnet into a magnet with further improved performance, the oxygen content in the alloy needs to decrease. However, if the oxygen content in the alloy is decreased, abnormal grain growth is likely to happen during the sintering process, resulting in a decreased squareness ratio or a substantial decline in coercivity. Since the oxides formed by the oxygen in the alloy prevent grains from growing, the decline of oxygen content in the alloy is likely to cause the abnormal grain growth.
Accordingly, a method is studied to enhance the magnetic properties by adding new elements in the R-T-B based sintered magnet containing Cu. In Patent Document 2, it is reported that Zr and/or Cr are/is added to provide a high coercivity and a high residual magnetic flux density.
Similarly, Patent Document 3 has reported to uniformly disperse and precipitate a finely divided ZrB compound, NbB compound or HfB compound in an R-T-B based rare earth permanent magnet containing Co, Al and Cu and further containing Zr, Nb or Hf. In this way, the grain is prevented from growing during the sintering process so as to improve the magnetic properties and the sintering temperature range.
Recently, in order to decrease the amount of the heavy rare earth elements with rare resource in use such as Dy or Tb, a method is adopted in which the main phase grains in the R-T-B based sintered magnet are micronized to improve the coercivity. However, if the main phase grains in the sintered magnet are to be micronized, the finely pulverized powder of raw materials has to be reduced in particle size. If the particle size of the finely pulverized powder is reduced, the abnormal grain growth tends to occur during the sintering process. Thus, when the finely pulverized powder with a small particle size is used as the raw material, the sintering temperature needs to be a low temperature so that a sintering process is performed for a relative long time, resulting in a substantial decline in productivity. As a method in which the finely pulverized powder with such a small particle size is used and the sintering process is conducted under the same conditions as those in the conventional method, it is considered that the amount of Zr to be added as the element having a high effect in preventing from abnormal grain growth has to be further increased. However, with the increase of the addition amount of Zr, technical problems will arise that the residual magnetic flux density decreases and good properties aimed to provide cannot be obtained.